# A question of notation

In a book I am reading I see the following:

$$m(B_n) \uparrow m(A)\quad \textrm{if}\, B_1 \subset B_2 \subset \dots\, \textrm{and}\, \bigcup_{n=1}^{\infty} B_n = A$$

What does the up arrow signify? m(A) denotes the lebesgue measure of A if that helps.